Interesting read here. (http://flowingdata.com/2008/04/29/why-should-engineers-and-scientists-care-about-color-and-design/) This is the interesting bit:

To represent continuous metrics, it was found that use of hue is poor while it's easier for us to interpret continuous changes (i.e. magnitude information) with saturation and luminescence. For interval data with a threshold, you can use multiple colors.

This stacks up with hypsometric colour schemes: Different shades of blue for bathymetrics, deep green to light green for lowlands, dark brown to light brown for mountains ending with white for peaks.